Photovoltaic inverters are configured to convert DC power produced via a photovoltaic output device (such as a photovoltaic module, a photovoltaic string or a photovoltaic array composed of the photovoltaic modules) into AC power. According to the known output characteristics of the photovoltaic module, the photovoltaic module has a weak output load capacity and fast power changing in a case of insufficient light (for example, on a morning, evening or cloudy day). In such situations, the output voltage of the photovoltaic output device may become lower due to the output power changing of the photovoltaic output device and the loading effect of the photovoltaic inverter once the photovoltaic inverter is connected in the electrical grid, although the open circuit voltage of the photovoltaic output device reaches a needed feed-in-grid DC voltage of the photovoltaic inverter connected to the photovoltaic output device. In a case that the output voltage of the photovoltaic output device cannot maintain the grid connection of the inverter, the photovoltaic inverter is disconnected from the electrical grid and shut down, which is generally referred to as a hiccup phenomenon. The hiccup phenomenon may occur repeatedly in a case that the photovoltaic output device has the weak output load capacity. Hence, the mechanical life of grid connection switches (such as relays or AC contactors) is shortened, which results in a short operation life of the photovoltaic system, and low reliability of the photovoltaic system.
At present, the following conventional technical solution is provided, in order to avoid the hiccup phenomenon of the photovoltaic system. A switched DC load device (as shown in FIG. 1) is added at a DC side of the photovoltaic system; a DC load is put into operation in a case that the photovoltaic output device outputs weak power or the photovoltaic inverter is disconnected from the electrical grid; the power output by the photovoltaic output device is consumed by the DC load to pull low the output voltage of the photovoltaic output device, so that the photovoltaic inverter is prevent from being connected to the electrical grid due to insufficient DC voltage. After the DC load is put into operation, in a case that the output voltage of the photovoltaic output device still reaches the needed feed-in-grid DC voltage of the photovoltaic inverter, the photovoltaic output device outputs enough power to maintain the grid connection of the photovoltaic inverter. The DC load device is switched off after the photovoltaic inverter is connected to the electrical grid.
However, an additional DC load is needed in the conventional technical solution stated above, therefore, the cost of the photovoltaic system is increased, and the volume of the photovoltaic system is increased since the DC load takes up space.